radiative and climate impacts of a large volcanic eruption ... · stratospheric sulfur injections...

Atmos. Chem. Phys., 16, 305–323, 2016

www.atmos-chem-phys.net/16/305/2016/

doi:10.5194/acp-16-305-2016

© Author(s) 2016. CC Attribution 3.0 License.

Radiative and climate impacts of a large volcanic eruption during

stratospheric sulfur geoengineering

A. Laakso1, H. Kokkola1, A.-I. Partanen2,3, U. Niemeier4, C. Timmreck4, K. E. J. Lehtinen1,5, H. Hakkarainen6, and

H. Korhonen2

1Finnish Meteorological Institute, Atmospheric Research Centre of Eastern Finland, Kuopio, Finland2Finnish Meteorological Institute, Climate Research, Helsinki, Finland3Department of Geography, Planning and Environment, Concordia University, Montréal, Québec, Canada4Max Planck Institute for Meteorology, Hamburg, Germany5Department of Applied Physics, University of Eastern Finland, Kuopio campus, Kuopio, Finland6A. I. Virtanen Institute for Molecular Sciences, University of Eastern Finland, Kuopio, Finland

Correspondence to: A. Laakso ([email protected])

Received: 18 June 2015 – Published in Atmos. Chem. Phys. Discuss.: 12 August 2015

Revised: 21 December 2015 – Accepted: 22 December 2015 – Published: 18 January 2016

Abstract. Both explosive volcanic eruptions, which emit sul-

fur dioxide into the stratosphere, and stratospheric geoengi-

neering via sulfur injections can potentially cool the climate

by increasing the amount of scattering particles in the at-

mosphere. Here we employ a global aerosol-climate model

and an Earth system model to study the radiative and climate

changes occurring after an erupting volcano during solar ra-

diation management (SRM). According to our simulations

the radiative impacts of the eruption and SRM are not addi-

tive and the radiative effects and climate changes occurring

after the eruption depend strongly on whether SRM is con-

tinued or suspended after the eruption. In the former case,

the peak burden of the additional stratospheric sulfate as well

as changes in global mean precipitation are fairly similar re-

gardless of whether the eruption takes place in a SRM or non-

SRM world. However, the maximum increase in the global

mean radiative forcing caused by the eruption is approxi-

mately 21 % lower compared to a case when the eruption

occurs in an unperturbed atmosphere. In addition, the recov-

ery of the stratospheric sulfur burden and radiative forcing

is significantly faster after the eruption, because the eruption

during the SRM leads to a smaller number and larger sulfate

particles compared to the eruption in a non-SRM world. On

the other hand, if SRM is suspended immediately after the

eruption, the peak increase in global forcing caused by the

eruption is about 32 % lower compared to a corresponding

eruption into a clean background atmosphere. In this sim-

ulation, only about one-third of the global ensemble-mean

cooling occurs after the eruption, compared to that occur-

ring after an eruption under unperturbed atmospheric con-

ditions. Furthermore, the global cooling signal is seen only

for the 12 months after the eruption in the former scenario

compared to over 40 months in the latter. In terms of global

precipitation rate, we obtain a 36 % smaller decrease in the

first year after the eruption and again a clearly faster recovery

in the concurrent eruption and SRM scenario, which is sus-

pended after the eruption. We also found that an explosive

eruption could lead to significantly different regional climate

responses depending on whether it takes place during geo-

engineering or into an unperturbed background atmosphere.

Our results imply that observations from previous large erup-

tions, such as Mount Pinatubo in 1991, are not directly ap-

plicable when estimating the potential consequences of a vol-

canic eruption during stratospheric geoengineering.

1 Introduction

Solar radiation management (SRM) by injecting sulfur to

the stratosphere is one of the most discussed geoengineer-

ing methods, because it has been suggested to be affordable

and effective and its impacts have been thought to be pre-

dictable based on volcanic eruptions (Crutzen, 2006; Rasch

et al., 2008; Robock et al., 2009; McClellan et al., 2012).

Published by Copernicus Publications on behalf of the European Geosciences Union.

306 A. Laakso et al.: Radiative and climate impacts of a large volcanic eruption

Stratospheric sulfur injections could be seen as an analogue

of explosive volcanic eruptions, during which large amounts

of sulfur dioxide (SO2) are released into the stratosphere.

Once released, SO2 oxidizes and forms aqueous sulfuric acid

particles which can grow to large enough sizes (some hun-

dreds of nanometres) to efficiently reflect incoming solar ra-

diation back to space. In the stratosphere, the lifetime of the

sulfate particles is much longer (approximately 1–2 years)

than in the troposphere, and the cooling effect from sulfate

aerosols may last for several years, as has been observed af-

ter large volcanic eruptions, such as Mount Pinatubo in 1991

(Hansen et al., 1992; Robock, 2000; Stenchikov et al., 2009).

Stratospheric SRM would maintain a similar aerosol layer in

the stratosphere continuously and could therefore be used (at

least in theory) as a means to buy time for the greenhouse gas

emission reductions (Keith and MacMartin, 2015).

One concern in implementing stratospheric SRM is that

an explosive eruption could happen while SRM is being de-

ployed. While it is impossible to predict the timing of such

eruptions, large volcanic events are fairly frequent with three

eruptions in the 20th century suggested having volcanic ex-

plosivity index (VEI) value of 6, indicating substantial strato-

spheric injections (Santa María in 1902, Novarupta/Katmai

in 1912, and Pinatubo in 1991) (Robock, 2000). Thus it is

possible that a large volcanic eruption could happen during

SRM deployment, which would most likely be ongoing for

decades. Should this happen, it could lead temporarily to a

very strong global cooling effect when sulfate particles from

both SRM and the volcanic eruption would reflect solar ra-

diation back to space. While the climate effects of volcanic

eruptions into an unperturbed atmosphere have been inves-

tigated in many previous studies (see overview papers by

Robock, 2000, and Timmreck, 2012), they may be differ-

ent if a volcanic eruption took place during SRM. In the

unperturbed atmospheric conditions, the stratosphere is al-

most clean of particles, while during SRM there would al-

ready be a large amount of sulfate in the stratosphere prior

to the eruption. Thus, the temporal development of the vol-

canic aerosol size distribution and related to this the volcanic

radiative forcing under SRM conditions may behave very dif-

ferently.

Here we study the effects of a volcanic eruption during

SRM by using two Max Planck Institute models – i.e. the

general circulation model (GCM) MAECHAM5 (Giorgetta

et al., 2006) coupled to an aerosol microphysical module

HAM-SALSA (Bergman et al., 2012; Kokkola et al., 2008),

and the Max Planck Institute Earth System Model (MPI-

ESM) (Giorgetta et al., 2013). We investigate the simulated

characteristics of the stratospheric sulfur burden, radiative

forcing, and global and regional climate effects.

2 Methods

2.1 Model descriptions

The simulations were performed in two steps. In the first step,

we used the aerosol-climate model MAECHAM5-HAM-

SALSA to define global aerosol fields in scenarios with

stratospheric sulfur injections and/or a volcanic eruption. In

the second step, we prescribe the simulated stratospheric

aerosol fields from MAECHAM5-HAM-SALSA to MPI-

ESM, similar to Timmreck et al. (2010).

2.1.1 Defining aerosol fields with

MAECHAM5-HAM-SALSA

For the global aerosol simulation we use MAECHAM5-

HAM-SALSA. The atmospheric model MAECHAM5 is a

middle atmosphere configuration of ECHAM5, in which the

atmosphere is divided into 47 height levels reaching up to

∼ 80 km. MAECHAM5 is integrated with a spectral trun-

cation of 63 (T63), which corresponds approximately to a

1.9◦× 1.9◦ horizontal grid. The simulations were performed

with a time step of 600 s.

The aerosol module HAM is coupled interactively to

MAECHAM5 and it calculates aerosol emissions and re-

moval, gas and liquid phase chemistry, and radiative prop-

erties for the major global aerosol compounds of sulfate, or-

ganic carbon, black carbon, sea salt and mineral dust.

In the original ECHAM-HAM (Stier et al., 2005), the

aerosol size distribution is described with seven lognormal

particle modes with fixed standard deviations and is designed

to represent the tropospheric aerosol conditions. Therefore,

the width of the coarse mode is optimized for description of

sea salt and dust particles, and it does not perform well in

special cases like volcanic eruptions or SRM, when a fairly

monodisperse coarse mode of sulfate particles can form in

the stratosphere (Kokkola et al., 2009). Simulating strato-

spheric aerosols would then require narrower coarse mode

(see e.g. Niemeier et al., 2009) which on the other hand is

not appropriate for simulating tropospheric aerosols. Here

we chose to use a sectional aerosol model SALSA (Kokkola

et al., 2008), which has been previously implemented with

ECHAM-HAM (Bergman et al., 2012) and is used to cal-

culate the microphysical processes of nucleation, condensa-

tion, coagulation and hydration. SALSA does not restrict the

shape of the size distribution making it possible to simulate

both tropospheric and stratospheric aerosols with the same

aerosol model.

The default SALSA setup divides the aerosol number and

volume size distribution into 10 size sections, which are

grouped into three subregions (Fig. 1, left-hand panel, dis-

tribution a). In addition, it has 10 extra size sections to de-

scribe external mixing of the particles (Fig. 1, left hand panel,

distributions b and c). In order to keep the number of tracer

variables to the minimum, in the third subregion (coarse par-

Atmos. Chem. Phys., 16, 305–323, 2016 www.atmos-chem-phys.net/16/305/2016/

A. Laakso et al.: Radiative and climate impacts of a large volcanic eruption 307

Figure 1. Particle size sections and chemical species in aerosol

model SALSA. The left-hand panel illustrates the standard SALSA

set-up. The rows “a”, “b” and “c” denote the externally mixed parti-

cle distributions. Within each distribution and subregion, N denotes

number concentration and SU, OC, BC, SS and DU respectively

sulfate, organic carbon, black carbon, sea salt and dust masses,

which are traced separately. Within distributions “a” and “b” in sub-

region 3, only particle number concentration is tracked, and all par-

ticles are assumed to be sea salt in distribution “a” (N(SS)) and dust

in distribution “b” (N(DU)). In distribution “c” only number con-

centration (N(DU)) and water soluble fraction (WS) are traced. The

numbers at the bottom of each subregion illustrate the size sections

within that subregion. In our study, the third subregion is excluded

and the second subregion is broadened to cover subregion 3 size

sections (right-hand panel).

ticles) only a number concentration in each section is tracked

and thus the particle dry size is prescribed. This means that

the sulfate mass is not explicitly tracked in this region al-

though it is allowed to change the solubility of the dust par-

ticles (distribution c in Fig. 1). In addition, there is no coag-

ulation and condensation growth inside this third subregion,

although smaller particles and gas molecules can be depleted

due to collisions with particles in subregion 3. In standard

tropospheric conditions, this kind of description of the coarse

particles is sufficient and it saves computational time and re-

sources. However, when studying large volcanic eruptions or

stratospheric sulfur geoengineering, microphysical process-

ing of an aerosol by a large amount of stratospheric sulfur can

significantly modify also the size distribution of coarse par-

ticles during their long lifetime (Kokkola et al., 2009). With

the default setup, this processing cannot be reproduced ade-

quately. In addition, information on the sulfur mass in each

size section in the coarse size range is not available in the

default setup. Thus we modified the SALSA model to ex-

clude the third subregion and broadened the second subre-

gion to cover also the coarse-particle range, as is shown in

Fig. 1 (right-hand panel). This allows a better representation

of coarse particles in the stratosphere, but increases simula-

tion time by approximately 30 % due to an increased number

of the particle composition tracers.

In addition to the sulfur emissions from SRM and

from volcanic eruptions (described in Sect. 2.2), the

MAECHAM5-HAM-SALSA simulations include aerosol

emissions from anthropogenic sources and biomass burning

as given in the AEROCOM database for the year 2000 (Den-

tener et al., 2006). For sea spray emissions, we use a pa-

rameterization combining the wind-speed-dependent source

functions by Monahan et al. (1986) and Smith and Harri-

son (1998) (Schulz et al., 2004). Dust emissions are calcu-

lated online as a function of wind speed and hydrological pa-

rameters according to the Tegen et al. (2002) scheme. We do

not include volcanic ash emissions as it has been shown that

ash sediments within a few days after the eruption from the

stratosphere, and the area affected by the ash cloud is rela-

tively small (Guo et al., 2004a). The effect of fine ash on the

distribution of the volcanic cloud in the atmosphere is also

relatively small (Niemeier et al., 2009).

The MAECHAM5-HAM-SALSA simulations were car-

ried out with a free-running setup without nudging. Thus

the dynamical feedback resulting from the additional heat-

ing from increased stratospheric sulfate load was taken into

account. Global aerosol model studies of the Pinatubo erup-

tion (Timmreck et al., 1999; Aquila et al., 2012) showed

that the dynamic response to local aerosol heating has an

important influence on the initial dispersal of the volcanic

cloud. Performing non-interactive and interactive Pinatubo

simulations, these studies revealed that an interactive cou-

pling of the aerosol with the radiation scheme is necessary

to adequately describe the observed transport characteristics

over the first months after the eruption. Only the interac-

tive model simulations where the volcanic aerosol is seen

by the radiation scheme are able to simulate the observed

initial southward cross-equatorial transport of the cloud as

well as the aerosol lifting to higher altitudes. A further im-

provement of the interactive simulation is a reduced north-

ward transport and an enhanced meridional transport towards

the south, which is consistent with satellite observations. On

the other hand, not running the model in the nudged mode

means that the online emissions, of for example sea salt and

mineral dust that are sensitive to wind speed at 10 m height,

can differ significantly between the simulations. This can oc-

casionally have fairly strong local effects on the aerosol ra-

diative forcing. However, the global radiative forcing from

dust is small compared to the forcing from the volcanic erup-

tion and SRM. The radiative forcing resulting from aerosol

loadings was calculated using a double call of radiation (with

and without aerosols).

Because MAECHAM5-HAM-SALSA is not coupled to

the ocean model, the simulations presented below have been

done using fixed sea surface temperatures. All runs are pre-

ceded by a 2-year spin-up period followed by a 5-year simu-

lation period for the baseline scenarios (defined in Sect. 2.2)

and a 3-year simulation period for the sensitivity scenarios

(Appendix B). Only one MAECHAM5-HAM-SALSA sim-

ulation has been performed for each of the studied scenarios

to obtain the aerosol optical fields for the ESM simulations.

Only for Volc we have carried out a five-member ensemble

to address potential forcing uncertainties (Appendix A).

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2.1.2 Determining climate effects with MPI-ESM

In the second step, simulations to quantify the global and re-

gional climate effects of concurrent SRM and volcanic erup-

tion are performed with the Earth system model MPI-ESM

(Giorgetta et al., 2013). The model is a state-of-the-art cou-

pled 3-dimensional atmosphere–ocean–land surface model.

It includes the atmospheric component ECHAM6 (Stevens

et al., 2013), which is the latest version of the atmospheric

model ECHAM and whose earlier version is used in the first

step of this study. The atmospheric model was coupled to

the Max Planck Institute Ocean Model (MPIOM) (Jungclaus

et al., 2013). MPI-ESM also includes the land model JS-

BACH (Reick et al., 2013) and the ocean biochemistry model

HAMOCC (Ilyina et al., 2013). ECHAM6 was run with the

same resolution as in the first part of this study. We did not

include dynamical vegetation and carbon cycle in the simu-

lations.

In MPI-ESM, aerosol fields are prescribed. We used the

same tropospheric aerosols fields based on the Kinne et

al. (2013) climatology in all scenarios. In the stratosphere,

we use precalculated aerosol fields from the different sim-

ulations with MAECHAM5-HAM-SALSA. The aerosol ra-

diative properties were calculated based on monthly mean

values of the aerosol effective radius and the aerosol opti-

cal depth (AOD) at 550 nm. MPI-ESM uses a precalculated

look-up table to scale AOD at 550 nm to the other radiation

wavelengths based on the effective radius. Here MPI-ESM

assumes the size distribution to consist of a single mode,

which in most cases differs from the sectional size distribu-

tion in MAECHAM5-HAM-SALSA. This can lead to some-

what different radiative forcings between MAECHAM5-

HAM-SALSA and MPI-ESM. In our study this has been

seen as overestimation of both shortwave and longwave forc-

ing. Overestimation is slightly larger in LW-radiation and

thus warming effect of MPI-ESM is overestimated in MPI-

ESM compared to the simulations by ECHAM-HAM. Since

there is very little zonal variation in the monthly mean strato-

spheric aerosol fields, the zonal mean aerosol fields from

MAECHAM5-HAM-SALSA are used in MPI-ESM.

The atmospheric gas concentrations were fixed to year

2010 level, in accordance with the tropospheric aerosol fields

and land use maps. Year 2010 concentrations were also used

for methane, chlorofluorocarbon and nitrous oxide.

Experiments with a full Earth system model require a long

spin-up period as the ocean component needs centuries to

stabilize. We resolved this by restarting our 105-year-long

spin-ups from previously run Coupled Model Intercompar-

ison Project Phase 5 (CMIP5) simulations ending in year

2005. Since the aerosol and atmospheric gas concentrations

in our simulations differed slightly from the CMIP5 runs,

the 105 years of spin-up was not enough for the model

to reach a full steady state; there was a small warming

(0.3 K (100 yr)−1) also after spin-up period in both CTRL

and SRM simulations (see simulation details in Sect. 2.2).

This temperature change is nevertheless so small that it does

not affect our conclusions.

Since the initial state of the climate system can have a

significant effect on the climate impacts resulting from forc-

ing, we ran 10-member ensembles of 5-year duration for all

baseline scenarios with a volcanic eruption. To do this, we

first ran the model for 50 years after the spin-up and saved

the climate state after every 5 years. We then continued the

simulations from each of these saved climate states for fur-

ther 5 years with a volcanic eruption taking place in these

specific climate conditions. The obtained results were com-

pared to the corresponding 5-year period in the simulations

without a volcanic eruption (which were run continuously for

50 years).

2.2 Model experiments

We simulated altogether five baseline scenarios in order to

investigate the radiative and climate impacts of concurrent

SRM and a volcanic eruption. To better separate the effects of

SRM and the eruption, these scenarios included also simula-

tions with only SRM or only a volcanic eruption taking place.

The studied scenarios are listed in Table 1, and detailed be-

low. Three additional sensitivity simulations investigating the

sensitivity of the results to the geographical location and the

seasonal timing of the eruption are presented in Appendix B.

All the simulations with SRM assumed continuous in-

jections of 8 Tg (S) yr−1 of SO2 between 30◦ N and 30◦ S

and 20–25 km in the vertical. The injection strength of

8 Tg (S) yr−1 was chosen based on previously published

SRM studies and for example Niemeier et al. (2011) has

shown such injection rates to lead to all-sky global short-

wave radiative forcing of −3.2 to −4.2 W m−2 in ECHAM5-

HAM. This forcing is roughly comparable (but opposite in

sign) to forcing from doubling of CO2 from preindustrial

level. Such a strong SRM forcing could be considered real-

istic in view of the business-as-usual scenario of the Repre-

sentative Concentration Pathways (RCP8.5), which estimates

that without efforts to constrain the greenhouse gas emissions

the total radiative forcing from anthropogenic activities at the

end of the 21st century is roughly 8.5 W m−2 (IPCC, 2013).

All the simulations with a volcanic eruption assumed an ex-

plosive eruption releasing 8.5 Tg of sulfur to the stratosphere

(Niemeier et al., 2009; Guo et al., 2004b; Read et al., 1993).

This corresponds to the magnitude of the Mount Pinatubo

eruption in June 1991. In all of the volcanic eruption sce-

narios, sulfur was injected to the height of 24 km. The erup-

tion was always initiated on the first day of the month at

06:00 UTC and it lasted for 3 h.

The baseline scenarios summarized in Table 1 and de-

tailed below were simulated first with MAECHAM5-HAM-

SALSA, and then with MPI-ESM using the stratospheric

aerosol fields from MAECHAM5-HAM-SALSA simula-

tions. On the other hand, the sensitivity simulations in Ap-



Table 1. Studied sulfur injection and volcanic eruption scenarios.

Scenario Description

CTRL Control simulation with no SRM or explosive eruptions

SRM Injections of 8 Tg (S) yr−1 of SO2 between latitudes 30◦ N and 30◦ S between

20 and 25 km altitude

Volc Volcanic eruption at the site of Mount Pinatubo (15.14◦ N, 120.35◦ E) on 1 July.

8.5 Tg of sulfur (as SO2) injected at 24 km

SRM Volc Volcanic eruption during SRM. SRM suspended immediately after the eruption

SRM Cont Volcanic eruption during SRM. SRM still continued after the eruption

pendix B were run only with MAECHAM5-HAM-SALSA

because of the computational expense of the MPI-ESM code.

The control (CTRL) simulation included only standard

natural and anthropogenic aerosols with no SRM or explo-

sive eruptions, while the simulation SRM included SRM on

top of the background aerosol, but no volcanic eruption. All

the baseline scenarios which simulated a volcanic eruption

assumed a tropical eruption at the site of Mount Pinatubo

(15.14◦ N, 120.35◦ E), where a real explosive eruption took

place in summer 1991. We simulated a July eruption at this

site both in background conditions (simulation Volc) and dur-

ing SRM (simulation SRM Volc and SRM Cont). Due to

safety and economic considerations, it might be that SRM is

suspended at some point after the eruption. When this would

happen depends on several factors (decision-making process,

magnitude/timing of volcano). Here we study cases where

SRM was suspended immediately after the eruption (SRM

Volc) and we also simulated a scenario where SRM was con-

tinued despite the eruption (SRM Cont). The purpose of the

latter simulation was also to study how additive the radia-

tive effects of volcanic eruption and solar radiation manage-

ment are. It should be noted that if the SRM injections are

suspended after a volcanic eruption, the injections should be

restarted after some time from the eruption to prevent abrupt

warming. However, we do not simulate the restart of SRM

injections in this study.

3 Results

3.1 Microphysical simulations of volcanic eruption and

SRM compared to the measurements and previous

studies

A comparison of the Volc simulation against observations

of the Pinatubo 1991 eruption shows that the model repro-

duces well the temporal behaviour of particle effective ra-

dius after a tropical eruption (Fig. A1b in Appendix A).

The model overestimates sulfate burden compared to those

retrieved from the HIRS satellite observations (Baran and

Foot, 1994) during the first 12 months after the eruption

(Fig. A1 in Appendix A). There are several previous global

model studies that where evolution of stratospheric aerosols

following Pinatubo eruption has been investigated. Many of

these shows similar sulfate burden than in the our study and

overestimation of sulfate burden compared to the HIRS data

(Niemeier et al., 2009; English et al., 2012; Dhomse et al.,

2014; Sheng et al., 2015). This comparison between the lim-

ited set of the observational data and with other modelling

studies gives us confidence that the new MAECHAM5-

HAM-SALSA set-up simulates aerosol loads and proper-

ties consistent to observations under high stratospheric sulfur

conditions.

We first looked at the aerosol burdens and the radia-

tive impacts of a tropical volcanic eruption and SRM sepa-

rately based on the MAECHAM5-HAM-SALSA runs (sim-

ulations Volc and SRM, respectively). The maximum strato-

spheric sulfate burden after the volcanic eruption (Volc) is

8.31 Tg (S). 75 % of the erupted SO2 is oxidized during

2 months after the eruption and the global maximum of sul-

fate burden is reached 5 months after the eruption (Fig. 2a,

black solid line). After this, the burden starts to decline

rapidly, but remains above the level that was simulated prior

to the eruption for approximately 4 years. On the other hand,

continuous geoengineering with 8 Tg (S) yr−1 (SRM) leads

to the global stratospheric sulfate burden of 7.8 Tg (S) with

only little variation in time (Fig. 2a, dashed black line). The

total sulfur amount (SO2 and sulfate) in the stratosphere is

8.8 Tg (S) which indicates the average sulfur lifetime (sulfur

burden divided by the amount of the injected sulfur) in the

stratosphere to be 1.1 years. As previous studies have shown,

the lifetime of sulfur is strongly dependent on the injection

area and height, and the amount of injected sulfur. Some of

the studies have shown a lifetime of clearly less than a year

for the comparable magnitude of injected sulfur, when sulfur

is injected at a lower height than in our study (Heckendorn et

al., 2009; Pierce et al., 2010; Niemeier et al., 2011; English

et al., 2012), slightly under a year when sulfur is injected

at the same height as here (Heckendorn et al., 2009; Pierce

et al., 2010), and over a year when sulfur is injected higher

(Niemeier et al., 2011). Thus, overall our results are in good

agreement with the previous studies.

The maximum clear-sky shortwave (SW) surface forcing

in the Volc simulation reaches −6.36 W m−2 (Fig. 2b), which

is close to the average global mean forcing of −6.00 W m−2



Figure 2. (a) Stratospheric sulfate burden and (b) global mean clear-sky shortwave radiative forcing at the surface in the different scenarios.

In addition, the dashed purple line represents the sum of SRM and Volc runs, and is shown for comparison.

in the SRM simulation, as could be expected based on the

similar maximum and steady-state sulfate burdens, respec-

tively (Fig. 2a). In the presence of clouds, the change in

SW all-sky flux in SRM is smaller (−3.38 W m−2) than in

clear-sky conditions. Radiative forcing from the SRM is in

agreement with previous studies where the forcing effect

has been studied with climate models including an explicit

aerosol microphysics description. For example, Niemeier et

al. (2011) showed all-sky SW radiative forcings from −3.2

to −4.2 W m−2 for 8 Tg (S) yr−1 injection, and Laakso et

al. (2012) a forcing of −1.32 W m−2 for 3 Tg (S) injec-

tion. On the other hand, Heckendorn et al. (2009) simu-

lated a clearly smaller radiative forcing of −1.68 W m−2 for

10 Tg (S) injection.

The shortwave radiative effect (−6.00 W m−2) from the

sulfate particles originating from SRM is concentrated rela-

tively uniformly between 60◦ N and 60◦ S (not shown). SRM

leads also to a 0.73 W m−2 all-sky longwave radiative forc-

ing which is concentrated more strongly in the Tropics than

in the midlatitudes and polar regions. In the case of the vol-

canic eruption (Volc), forcing is distributed between 30◦ N

and the Equator for the first 4 months after the eruption. Af-

ter that, forcing is concentrated more to the midlatitudes than

the low latitudes in both hemispheres. It should be noted,

however, that the initial state of the atmosphere and local

winds over the eruption area at the time of the eruption can

have a large impact on the distribution of sulfur released

from a short-duration eruption. This can be seen for example

in Fig. A2, which illustrates the hemispheric sulfur burdens

from five different ensemble members of the Volc simulation

(see Appendix A for details). As an example, in one of the

ensemble simulations, burden is concentrated much more in

the Northern Hemisphere (NH) (peak value 6.7 Tg (S)) than

in the Southern Hemisphere (SH) (2.2 Tg (S)). This leads to

northern and southern hemispheric peak values of clear-sky

forcings of −8.18 and −3.72 W m−2, respectively. However,

in another ensemble member sulfate is distributed more uni-

formly between the hemispheres (4.8 and 3.7 Tg (S) in the

NH and SH, respectively) resulting in clear-sky peak forcing

of −6.04 W m−2 in the north and −6.35 W m−2 in the south.

(In the analysis above (e.g. Fig. 2), we have used simulation

Volc4 from Appendix A, since it resembles most closely the

5-member ensemble mean in terms how sulfate is distributed

between the hemispheres.)

3.2 Burden and radiative effects of concurrent volcanic

eruption and SRM – results of aerosol

microphysical simulations

Next we investigated whether the radiative impacts from a

volcanic eruption taking place during SRM differs from the

sum of volcanic-eruption-only and SRM-only scenarios dis-

cussed in Sect. 3.1. In order to do this, we compared the

SRM-only (SRM) and volcanic-eruption-only (Volc) simu-

lations with two scenarios of concurrent eruption and SRM:

SRM Volc where SRM is suspended immediately after the

eruption, and SRM Cont where SRM is continued after the

eruption. The magnitude, timing and location of the eruption

were assumed the same as in Volc simulation.

Figure 2 shows the stratospheric sulfur burden and

the global clear-sky radiative forcing from the four

MAECHAM5-HAM-SALSA runs. It is evident that both the

stratospheric sulfate burden and the global shortwave radia-

tive forcing reach a maximum value and recover back to pre-

eruption level clearly faster if the volcanic eruption happens

during SRM than in stratospheric background conditions, as

can be seen by comparing the scenario of volcanic erup-

tion concurrent with SRM (solid blue and red lines) to the

sum of eruption-only and SRM-only scenarios (dashed pur-

ple line). This is the case especially when SRM is suspended

immediately after the eruption (simulation SRM Volc). In

this case, in our simulation set-up, it takes only 10 months

for the stratospheric sulfate burden and the global radiative

effect to recover to the state before the volcanic eruption. On

the other hand, if the eruption happens in stratospheric back-

ground conditions (Volc), it takes approximately 40 months

before the sulfate burden and the radiative effect return to

their pre-eruption values. In addition, the global SW radia-



tive forcing reaches a maximum value two months earlier

in SRM Volc than in Volc (Fig. 2b). In comparison to the

level before the eruption, the peak increase in radiative forc-

ing is 32 % smaller in SRM Volc (−4.30 W m−2) than in Volc

(−6.36 W m−2).

The first, somewhat trivial reason for lower and shorter-

lasting radiative forcing in SRM Volc is that because SRM is

suspended immediately after the eruption, the stratospheric

sulfur load will recover from both the volcanic eruption and

SRM. If the stratospheric background sulfur level is not up-

held by continuous sulfur injections as before the eruption,

the sulfur burden will return back to the pre-eruption con-

ditions within less than a year after the eruption. However,

the different responses to a volcanic eruption during back-

ground (Volc) and SRM (SRM Volc) conditions cannot be

explained only by suspended SRM injections. This can be

seen in Fig. 2a in scenario SRM Cont (solid red line) where

geoengineering is continued after the volcanic eruption: also

in this case the lifetime of sulfate particles is shorter than in

Volc. There is a similar increase in the sulfate burden in the

first 10 months after the eruption in the Volc and SRM Cont

scenarios as is seen by comparing the red and purple lines in

Fig. 2; here the purple dashed line shows the calculated sum

of the effects from separate simulations of Volc and SRM.

This scales the Volc simulation to the same start level as SRM

Cont. After the first 10 months the sulfate burden starts to

decrease faster in the SRM Cont scenario and is back to the

level prior to the eruption after 20 months from the eruption,

compared with ∼ 40 months in the Volc run. The difference

between the two scenarios can be seen even more clearly in

the shortwave radiative forcing (Fig. 2b). When the volcano

erupts during SRM, the contribution of the eruption to the

forcing is lower immediately after the eruption than after the

eruption in Volc and the peak increase in global mean radia-

tive forcing compared the pre-eruption level is 21 % lower in

SRM Cont (−5.04 W m−2) than in Volc (−6.36 W m−2).

The reason for these findings is that the initial stratospheric

aerosol load is significantly different when the volcanic erup-

tion occurs during stratospheric sulfur geoengineering than

under background conditions. If a volcano erupts concur-

rently with SRM, sulfur from the eruption does not only form

new particles but also condenses onto pre-existing particles.

Furthermore, the new small particles that are formed after the

eruption coagulate effectively with the existing larger parti-

cles from the SRM injections. This means that a situation

develops where there are fewer but larger particles compared

to a case without SRM. The increased particle size can also

be seen in Fig. 3 which shows the effective radius in the SRM

injection area. These larger particles in SRM Volc and SRM

Cont have higher gravitation settling velocities and sediment

faster. Thus, about 30 months after the eruption the effective

radius in SRM Volc becomes even smaller than in simula-

tion Volc, when larger particles have sedimented out of the

atmosphere in SRM Volc. Figure 2 indicates the impact on

the radiative forcing. SW scattering gets less effective with

0 10 20 30 40 500

0.1

0.2

0.3

0.4

0.5

0.6

Month after the eruption

Effe

ctiv

e ra

dius

(μm

)

SRM

Volc

SRM Volc

SRM Cont

Figure 3. Mean effective radius in the different scenarios between

20◦ N and 20◦ S latitudes and between 20 and 25 km altitude levels.

increasing particle size (Pierce et al., 2010) and, although the

stratospheric sulfur burden is the same in the first months af-

ter the eruption in SRM Cont and in the sum of Volc and

SRM, there is a clear difference in the radiative forcing. This

indicates that the number-to-mass ratio of particles is smaller

in SRM Cont than in the calculated sum from Volc and SRM.

Additional sensitivity simulations with MAECHAM5-

SALSA discussed in more detail in Appendix B show that

the season when the tropical eruption occurs defines how

sulfate from the eruption is distributed between the hemi-

spheres. An eruption in January leads to a larger sulfur bur-

den in the Northern Hemisphere than an eruption in July

(Toohey et al., 2011; Aquila et al., 2012). This conclusion

holds also if the eruption occurs during geoengineering, at

least in cases where SRM is implemented evenly to both

hemispheres. In the case of an eruption outside the Tropics,

the season of the eruption can have a large impact on the

magnitudes of both the sulfate burden and the global radia-

tive forcing. Therefore it very likely has an impact also on

the regional climates, which further defines when and where

suspended stratospheric sulfur injections should be restarted.

However, due to the computational expense of the fully cou-

pled MPI-ESM, we limit our analysis of the climate impacts

below only to the baseline scenarios. It should be noted that

the impact after concurrent volcanic eruption and SRM may

depend also on the altitude at which sulfur is released. In-

creasing the injection height increases the lifetime of sulfate

(Niemeier and Timmreck, 2015). If sulfur from the eruption

is released at the same altitude where SRM sulfur resides, it

might lead to locally larger sulfur concentration and there-

fore to larger particles compared to a case when sulfur from

the eruption is released below the SRM sulfate layer. De-

pendent on the geographical location this volcanic sulfur can

still reach the SRM layer, e.g in the case of tropical eruption

with the ascending branch of the Brewer–Dobson circulation.

However, this happens on much longer timescales.



Figure 4. Global mean 2 m (a) temperature and (b) precipitation changes after the volcanic eruption compared to the background condition

(black line) and during solar radiation management (blue and red lines). Solid lines are mean values of the ten members of the ensemble

simulations. The maximum and minimum values of the ensemble are depicted by shaded areas.

3.3 Climate effects from concurrent volcanic eruption

and SRM – results of ESM simulations

In this section we investigate how the radiative forcings sim-

ulated for the different scenarios in section 3.2 translate into

global and regional climate impacts. For this purpose, we im-

plemented the simulated AOD and effective radius of strato-

spheric sulfate aerosol from MAECHAM5-HAM-SALSA to

MPI-ESM, similar to Timmreck et al. (2010).

Figure 4a shows the global mean temperature change com-

pared to the pre-eruption climate. Simulation Volc (black

line) leads to cooling with an ensemble mean peak value of

−0.45 K reached 6 months after the eruption. On average,

this cooling impact declines clearly more slowly than the ra-

diative forcing after the eruption (shown in Fig. 2b): 1 year

after the eruption the radiative forcing was 64 % of its peak

value, and subsequently 17 and 8 % of the peak value 2 and

3 years after the eruption. On the other hand, the ensemble

mean temperature change 1 year after the eruption is 84 % of

the peak value. Subsequently, 2 and 3 years after the erup-

tion the temperature change is still 53 and 30 % of the peak

value. It should be noted, however, that the variation in tem-

perature change is quite large between the 10 climate sim-

ulation ensemble members (±0.67 K compared the mean of

the ensemble). In fact, in some of the ensemble members the

pre-eruption temperature is reached already approximately

15 months after the eruption.

Figure 4a also shows that on average a volcanic erup-

tion during continued SRM (simulation SRM Cont, red line)

leads to on average 33 % smaller cooling for the next 3 years

after the eruption than under unperturbed atmospheric condi-

tions. If SRM is suspended (SRM Volc), the maximum value

of the global cooling is only about one-third (i.e. less than

0.14 K at maximum for the ensemble mean) compared to an

eruption to the non-geoengineered background stratosphere

(simulation Volc). This is consistent with the clearly smaller

radiative forcings predicted for the eruption during SRM than

in the background atmospheric conditions (Fig. 2b). Simi-

lar to Volc simulation, the global mean temperature is lower

compared to the pre-eruption level and even radiative forcing

has levelled off. In SRM Volc scenario the global mean short-

wave radiative forcing from the sulfate particles has reached

the pre-eruption level after 10 months from the eruption but

there would be still some global cooling after 12 months from

the eruption. Our simulations indicate that if SRM is sus-

pended but not restarted, there is fast warming compared to

the pre-eruption temperature within the first 20 months after

the eruption.

Figure 5 depicts the regional surface temperature changes

simulated in the different scenarios. Geoengineering alone

(SRM) would lead to global ensemble mean cooling of

−1.35 K compared to the CTRL case. As Fig. 5a shows,

cooling is clearly stronger in the Northern Hemisphere

(−1.65 K) than in the Southern Hemisphere (−1.05 K). The

strongest regional cooling is seen in the northern high lati-

tudes (regional average of −2.2 K north of 50◦ N). The small-

est cooling effect, or even slight warming, is predicted over

the southern oceans. These general features are consistent

with the GeoMIP multimodel intercomparison when only the

impact of SRM (and not of combined SRM and CO2 in-

crease) is considered: Kravitz et al. (2013a) show a very sim-

ilar decrease in polar temperature when subtracting temper-

ature change under increased CO2 from the combined SRM

and CO2 increase results.

For the three volcanic eruption scenarios we concentrate

on the regional climate impacts during the first year after

the eruption. Figure 5b shows the 1-year-mean temperature

anomaly at the surface after a volcanic eruption into the un-

perturbed background stratosphere in simulation Volc. As

expected, the cooling impact from the volcanic event over

the first year following the eruption is clearly smaller than

that from continuously deployed SRM. While there are some

similar features in the temperature change patterns between

Fig. 5a and b (such as more cooling in the Northern than in



Figure 5. Ensemble mean change in annual mean 2 m temperature. (a) 50-year mean temperature change in SRM scenario; 1-year-mean

temperature change after the volcanic eruption in (b) Volc, (c) SRM Volc and (d) SRM Cont compared to the pre-eruption climate (CTRL

for SRM and Volc, and SRM for SRM Volc and SRM Cont). Hatching indicates a regions where the change of temperature is statistically

significant at 95 % level. Significance level was estimated using Student’s unpaired t test with a sample of 10 ensemble member means for

panels (b–d) and a sample of 50 annual means for panel (a). Note the different scale in panel (a).

the Southern Hemisphere, and warming in the southern Pa-

cific), clear differences also emerge, especially in NH mid-

and high latitudes where there is less cooling, and in some re-

gions even warming, after the eruption. During the first year

after the eruption, sulfate from the tropical eruption is mainly

concentrated at low latitudes where there is also strong solar

intensity and thus strong radiative effect from the enhanced

stratospheric aerosol layer. During the subsequent years, sul-

fate transport towards the poles causes stronger cooling also

in the high latitudes. The global yearly mean temperature

change is −0.34 K for the first year after the eruption, then

decreasing to a value of −0.30 for the second year from

the eruption. However, there is an increased temperature re-

sponse north of 50◦ N from the first year mean of −0.30 K

to the second year mean of −0.44 K. Even though there is

larger cooling at the midlatitudes in the second year after the

eruption, we see 0.06 K warming north of 75◦ N in the sec-

ond boreal winter (December–February) after the eruption.

Winter warming after a volcanic eruption has been seen also

in observations (e.g. Robock and Mao, 1992; Fischer et al.,

2007), though the current generation of CMIP5 models has

problems to reproduce the NH post-volcanic winter warming

pattern (Driscoll et al., 2012).

When the eruption takes place during geoengineering and

SRM injections are suspended (SRM Volc), the global 1-year

ensemble mean temperature change is only −0.09 K during

the first year after the eruption (Fig. 5c). This small global

impact is due to the fact that the anomaly in SW radiation

after the volcanic eruption is relatively small in magnitude

and only about 10 months in duration when geoengineer-

ing is suspended after the eruption (Fig. 2b). However, the

regional impacts are much stronger and show distinctly dif-

ferent patterns from those in Volc (Fig. 5b). Volc scenario

leads to 0.30 K cooling north from 50◦ N in the ensemble

mean, while there is small warming of 0.02 K in SRM Volc

after the first year from eruption. The warming is concen-

trated over the central areas of Canada, where the ensemble

mean temperature increase is more than 1 K, and over North

Eurasia, where the temperature increase is more than 0.5 K. It

should be noted, however, that in most parts of these regions

the warming signal is not statistically significant.

There are also differences in the southern hemispheric

temperatures between the different scenarios. While Volc

scenario leads to small −0.02 K mean cooling south of 50◦ S

in the first year after the eruption, there is a warming of

0.14 K in the SRM Volc scenario. In addition, over the Pa-

cific equatorial area the Volc scenario leads to a cooling of

more than −0.5 K while SRM Volc scenario leads to a warm-

ing of more than 0.5 K. These differences between Volc and

SRM Volc simulations imply that previous observations of

regional climate impacts after an explosive eruption, such

as Pinatubo in 1991, may not offer a reliable analogue for

the impacts after an eruption during SRM. It is important to

note, however, that just like there were some variations in the

global mean temperature between individual ensemble mem-

bers, there are also variations in regional changes between

the members. Variations are the largest over high latitudes,

while most of the individual ensemble members are in good



agreement at the low latitudes (hatching in Fig. 5), where the

change in temperature is the largest.

The main reason for the differences between Volc and

SRM Volc is that in the latter simulation the volcanic erup-

tion is preceded by SRM injections (providing a baseline

stratospheric sulfate load) which are suspended immediately

after the eruption. Thus, after the eruption the baseline sulfate

load starts decreasing, especially far away from the eruption

site, and, therefore, during the first year after the eruption

there are regions with a positive radiative forcing compared

to the pre-eruption level.

We also find that there could be regional warming in some

regions after the volcanic eruption even if the SRM injec-

tions were still continued (Fig. 5d, SRM Cont). This warm-

ing is concentrated in the high latitudes and areas with rel-

atively little solar shortwave radiation but with large strato-

spheric particles capable of absorbing outgoing longwave ra-

diation. The warming is strongest in the first post-eruption

boreal winter when some areas over Canada, Northeast Eu-

rope and western Russia experience over 0.5 K warming (not

shown). Such significant regional warming means that the

ensemble mean temperature change north of 50◦ N during

the first post-eruption winter is only −0.05 K. In some parts

of the Southern Ocean a volcanic eruption could enhance the

warming signal caused already by SRM (Fig. 5a).

It is also worth noting that the stratospheric sulfur geoengi-

neering with 8 Tg (S) yr−1 itself leads only to −1.35 K global

temperature change in our simulations. Such weak response

is likely at least partly due to the radiation calculations in

MPI-ESM, which assume a single modal particle size dis-

tribution (see Sect. 2.1.2 for details). Compared to a more

flat size distribution simulated by the sectional approach of

MAECHAM5-HAM-SALSA, this assumption leads to an

overestimation longwave (LW) AOD which is calculated

from 550 nm AOD. This in turn leads to an overestima-

tion of the longwave radiative forcing (0.7 W m−2 for SRM)

while the shortwave forcing is less affected (−0.2 W m−2 for

SRM). However, this does not affect the conclusions of this

study.

In addition to the changes in surface temperature, volcanic

eruptions will also lead to changes in precipitation. Figure 4b

shows the global mean precipitation change after a volcanic

eruption in the three scenarios. There is a similar decrease in

the precipitation in all volcanic scenarios during the first 5

months after the eruption. Thereafter there is a similar slow

increase in the global mean precipitation in the simulations

Volc and SRM Cont but a clearly faster increase in SRM

Volc. This faster increase would also, about 1 year after the

eruption, lead to a higher global ensemble mean precipitation

compared to the pre-eruption climate.

The global 1-year mean precipitation change is 0.036,

0.023 and 0.031 mm day−1 for Volc, SRM Volc and SRM

Cont respectively for the first year after the eruption. Earlier

studies (Bala et al., 2008; Kravitz et al., 2013a, b; Niemeier et

al., 2013) have already shown that geoengineering leads to a

reduction in the global precipitation compared to the climate

without geoengineering. In our SRM simulation, we obtain

a precipitation reduction of 0.11 mm day−1 (2.8 %), which is

clearly larger than the impact after the volcanic eruption.

The stratospheric sulfate affects precipitation via two cli-

mate system responses. The first one is the rapid adjustment

(fast response) due to atmospheric forcing, such as change

in solar irradiance, on a short timescale. The second one is

the feedback response (slow response) due to temperature

changes (Bony et al., 2013; Ferraro et al., 2014; Fuglestvedt

et al., 2014; Kravitz et al., 2013b). The signals from both of

these responses can be seen in Fig. 4b, especially in the sim-

ulation SRM Volc (blue line). During the first months after

the eruption, the precipitation drops relatively rapidly which

corresponds well with the rapid change in the radiative forc-

ing (Fig. 2b); at the same time, the temperature change in

SRM Volc is less steep (Fig. 4a). This implies that in the first

months following the eruption, the precipitation change is

more affected by the change in the radiation than the change

in the temperature. On the other hand, after 2 years from the

eruption there is only small SW radiative effect left from the

eruption (and the SRM prior to eruption) but there is still a

decrease in the global mean precipitation. During this period,

precipitation is predominantly affected by the change in tem-

perature.

Figure 6 shows the regional precipitation changes in each

of the studied scenarios. The largest changes after geoengi-

neering (SRM) are seen in the tropical convective region

where SRM reduces the precipitation rate in large areas by as

much as 0.5 mm day−1 (Fig. 6a). This is in good agreement

with previous multi-model studies (Kravitz et al., 2013a).

In our simulations, an increase of the same magnitude in

the precipitation rate is predicted just north of Australia,

which has not been seen in previous model intercomparisons

(Kravitz et al., 2013a).

Although the precipitation patterns in SRM and Volc

are similar in low latitudes, differences are seen especially

in NH mid- and high latitudes where SRM shows clearly

larger reduction in precipitation. The zonal mean value is

−0.15 mm day−1 in both 50◦ north and south latitudes. In

these areas, there is clearly less evaporation in the SRM sce-

nario which is not seen in the first year after the volcanic

eruption (Volc) and which would lead to different precipita-

tion patterns. Similar to the temperature change, our simula-

tions indicate that a tropical volcanic eruption impacts pre-

cipitation patterns differently in unperturbed and SRM con-

ditions. In fact, a volcanic eruption during geoengineering

(SRM Volc and SRM Cont) leads to an opposite precipita-

tion change pattern than an eruption to the unperturbed atmo-

sphere (Volc) over the tropical area in the Pacific and Atlantic

(Fig. 6c and d). In these areas, a volcanic eruption during

SRM leads to the increase in the evaporation flux at the sur-

face during the first year after the eruption, whereas the evap-

oration flux decreases if the eruption takes place in unper-

turbed conditions. This is caused by different tropical tem-



Figure 6. Ensemble-mean precipitation change in (a) 50-year mean precipitation change in the SRM scenario. The change in 1 year mean

precipitation after the volcanic eruption in (b) Volc, (c) SRM Volc and (d) SRM Cont compared to the pre-eruption climate (CTRL for SRM

and Volc, and SRM for SRM Volc and SRM Cont). Panels (b–d) show the 1-year-mean temperature after the eruption. Panel (a) shows the

mean over the corresponding 1-year-periods as the other panels. Hatching indicates a regions where the change of precipitation is statistically

significant at 95 % level. Significance level was estimated using Student’s unpaired t test with a sample of 10 ensemble member means for

panels (b–d) and a sample of 50 annual means for panel (a).

perature responses between the simulations (Fig. 5). Com-

pared to the pre-eruption values, in simulations SRM and

Volc, equatorial sea surface temperature (SST) anomalies

(latitudes 0–10◦ N) are relatively colder than the SST anoma-

lies over latitudes 10–20◦ N. In simulation SRM, the differ-

ence in SST anomaly between these areas is −0.02 K and in

simulation Volc it is −0.05 K. On the other hand, in simula-

tions SRM Volc and SRM Cont, equatorial SST anomalies

are relatively warmer than those over latitudes 10–20◦ N. In

SRM Volc, the difference in temperature anomaly is 0.13 K

and in SRM Cont it is 0.05 K. However, these changes in pre-

cipitation are not significant and a larger ensemble would be

necessary for further detailed investigations. It should also be

noted that here we have studied an unrealistic scenario where

SRM is implemented without global warming. If warming

from increased greenhouse gases had been included in the

scenarios, the temperature gradient could be very different

in simulation SRM which could lead to different precipita-

tion patterns. There is also a large natural variability in the

precipitation rates and as the precipitation changes after the

eruption are relatively small, our results are statistically sig-

nificant only in a relatively small area (hatching in Fig. 6).

4 Summary and conclusions

We have used an aerosol microphysical model coupled to

an atmosphere-only GCM as well as an ESM to estimate

the combined effects of stratospheric sulfur geoengineering

and a large volcanic eruption. First, MAECHAM5-HAM-

SALSA was used to define the stratospheric aerosol fields

and optical properties in several volcanic eruption and SRM

scenarios. Following the approach introduced in Timmreck et

al. (2010) and Niemeier et al. (2013), these parameters were

then applied in the Max Planck Institute Earth System Model

(MPI-ESM) in order to study their effects on the temperature

and precipitation.

According to our simulations, climate responses to be ex-

pected after a volcanic eruption during SRM depend strongly

on whether SRM is continued or halted after the eruption. In

the former case, the peak additional forcing is about 21 %

lower and the global cooling 33 % smaller than compared

to an eruption taking place in non-SRM world. However, the

peak additional burden and changes in global mean precipita-

tion are fairly similar regardless of whether the eruption takes

place in a SRM or non-SRM world. On the other hand, if

SRM is stopped immediately after the eruption, the peak bur-

den is 24 % and forcing 32 % lower and reached earlier com-

pared to the case with unperturbed atmosphere. Furthermore,

the forcing from the eruption declines significantly faster, im-

plying that if SRM was stopped after the eruption, it would

need to be restarted relatively soon (in our scenario within

10 months) after the eruption to maintain the pre-eruption

forcing level.

In line with the burden and forcing results, the simulated

global and regional climate impacts were also distinctly dif-

ferent depending on whether the volcano erupts during SRM



or in the background stratospheric conditions. In the in-

vestigated scenarios, a Pinatubo-type eruption during SRM

caused a maximum global ensemble-mean cooling of only

0.14 K (assuming that SRM is paused after the eruption)

compared to 0.45 K in the background case. On the other

hand, the ensemble-mean decline in the precipitation rate

was 36 % lower for the first year after the eruption during

SRM than for the eruption under unperturbed atmospheric

conditions. Both the global mean temperature and the pre-

cipitation rate recovered to the pre-eruption level in about

1 year, compared to approximately 40 months in the back-

ground case. If SRM was continued despite the large volcanic

eruption, the global ensemble mean cooling was averagely

−0.19 K (−(0.31–0.02) K in individual ensemble member)

for 3 subsequent years after the eruption. This is only 67 %

of 3 subsequent years cooling after the eruption in normal

unperturbed atmospheric conditions, when global ensemble

mean cooling was −0.28 K (−(0.49–0.15) K).

In terms of the regional climate impacts, we found cool-

ing throughout most of the Tropics regardless of whether the

eruption took place during SRM or in the background condi-

tions, but a clear warming signal (up to 1◦ C) in large parts of

the mid- and high latitudes in the former scenario. While it

should be noted that the regional temperature changes were

statistically significant mostly only in the Tropics, the declin-

ing stratospheric aerosol load compared to the pre-eruption

level (as a result of switching off SRM after the eruption) of-

fers a plausible physical mechanism for the simulated warm-

ing signal in the mid- and high latitudes. On the other hand,

the largest regional precipitation responses were seen in the

Tropics. Interestingly, the sign of the precipitation change

was opposite in SRM Volc and SRM Cont than in the Volc

and SRM in large parts of the tropical Pacific. We attribute

this difference to a clearly weaker tropical cooling, or in

some areas even a slight warming, in the former scenario

leading to an increased evaporation in the first year following

the eruption.

Based on both the simulated global and regional re-

sponses, we conclude that previous observations of explo-

sive volcanic eruptions in stratospheric background condi-

tions, such as the Mount Pinatubo eruption in 1991, are likely

not directly applicable to estimating the radiative and climate

impacts of an eruption during stratospheric geoengineering.

The global mean temperature and precipitation decline from

the eruption can be significantly alleviated if the SRM is

switched off after the eruption; however, large regional im-

pacts could still be expected during the first year following

the eruption.



Appendix A: Evaluation of the model: Pinatubo

eruption 1991, comparison between model and

measurements

This is the first study where ECHAM5-HAM-SALSA has

been used to simulate aerosol processes in the stratosphere.

To ensure that the model can be applied for simulation

of high aerosol load in the stratosphere, we evaluated the

model’s ability to reproduce the response of the stratospheric

aerosol layer to the Mount Pinatubo eruption in 1991. We

simulated the Pinatubo eruption with MAECHAM5-HAM-

SALSA making a five-member ensemble initiated on 1 July

(see simulation Volc in Sect. 2.2 for details). In these simu-

lations, we first used the same 2-year spin-up for all ensem-

ble members. After the spin-up, the model was slightly per-

turbed by a very small change in a model tuning parameter

and then run freely for 6 months, in order to create different

atmospheric states for the volcano to erupt into. Only after

this was the volcanic eruption triggered in the model. Sim-

ulated sulfur burdens and particle effective radii were com-

pared against observations from satellite (HIRS) (Baran and

Foot, 1994) and lidar measurements (Ansmann et al., 1997),

respectively.

Figure A1 shows that the model results are in general

in good agreement with the observations. For example, the

model correctly indicates that the oxidation of SO2 and for-

mation of sulfate particles is very fast right after the eruption.

However, the simulated sulfate burden peaks at higher values

than the observations after which sulfur burden decreases be-

low observed values approximately 1 year after the eruption.

This has been seen also in previous studies (e.g. English et

al., 2013, and Niemeier et al., 2009). English et al. (2013)

suggest that this might be because aerosol heating was not

included their model. Our model includes the aerosol heat-

ing effect and still underestimates the burden. This might be

due to poleward transport at the stratosphere which is over-

estimated in the model (Niemeier et al., 2009).

In all of the ensemble members the effective radius is gen-

erally overestimated during months 3–8 after the eruption,

although there is also large variation in the measured values

(Fig. A1b). The simulated maximum value for the effective

radius is reached 3–4 months earlier than in observations.

After 8 months from the eruption results from all the model

simulations are in good agreement with observations.

One possible explanation for the larger burden and effec-

tive radius in the model could be that the amount of erupted

sulfur is overestimated in the model compared to the real

Pinatubo eruption. Recent global stratospheric aerosol stud-

ies indicate a much better agreement with observations if

they assume a smaller amount of the volcanic SO2 emission

of 5 to 7 Tg (S) (Dhomse et al., 2014; Sheng et al., 2015). An-

other possible explanation is that a larger proportion of sulfur

was removed from the stratosphere during the first months

after the eruption due the cross-tropopause transport out of

the stratosphere or the enhanced removal with ash and ice

cloud (Dhomse et al., 2014). Unfortunately, there is only a

limited amount of observations after the eruption of Pinatubo

which makes comparison between model results and obser-

vations difficult. However, our results here are similar to the

previous model studies (Niemeier et al., 2009; English et al.,

2012; Dhomse et al., 2014; Sheng et al., 2015).

There is some variation in the predicted peak burden and

effective radii between the five members of the ensemble

simulation (Fig. A1). This indicates that the results are de-

pendent on the local stratospheric conditions at the time of

the eruption. Depending on meridional wind patterns during

and after the eruption, the released sulfur can be distributed

in very different ways between the hemispheres. This can

be seen in Fig. A2 which shows the sulfate burdens after

the eruption separately in the northern and southern hemi-

spheres. As the figure shows, in simulation Volc1 over 70 %

of the sulfate from the eruption is distributed to the North-

ern Hemisphere, whereas in Volc5 simulation it is distributed

quite evenly to both hemispheres. These very different spa-

tial distributions of sulfate lead to the aerosol optical depth

(AOD) fields illustrated in Fig. A3. The AOD in the North-

ern Hemisphere is clearly higher in the Volc1 simulation

(panel a) than in the Volc5 simulation (panel b) for about

18 months after the eruption, whereas the opposite is true for

the Southern Hemisphere for approximately the first 2 years

following the eruption. These results highlight that when in-

vestigating the climate effects of a volcanic eruption during

SRM, an ensemble approach is necessary.



Figure A1. (a) Global SO2 (dashed lines) and particulate sulfate (solid lines) burdens after a simulated volcanic eruption in July compared to

sulfate observations from HIRS satellite after the 1991 Pinatubo eruption (black). (b) Zonal mean effective radius at 53◦ N latitude after the

simulated July eruption compared to lidar measurements at Laramie 41◦ N (dots) and Geesthacht 53◦ N (crosses) after the Pinatubo eruption

(Ansmann et al., 1997). In both panels the results are shown for altitude range 16–20 km. The different coloured lines show results from the

five members of the simulated ensemble (simulations Volc1, . . . , Volc5).

Figure A2. SO2 (dashed lines) and sulfate (solid lines) burden after the eruption on (a) Northern Hemisphere and (b) Southern Hemisphere.

Note different the scales on the y axes.

Figure A3. Zonal and monthly mean 550 nm aerosol optical depth after volcanic eruption in (a) Volc1 simulation and (b) Volc5 simulation.



Appendix B: Sensitivity simulations: location and

season of the eruption

B1 Description of sensitivity runs

We also performed a set of sensitivity simulations to inves-

tigate how the season and location of the volcanic eruption

during SRM impacts the global sulfate burden and radia-

tive forcing. The baseline scenario SRM Volc was compared

with three new simulations summarized in Table B1 and de-

tailed below. These sensitivity runs were performed only us-

ing MAECHAM5-HAM-SALSA due to the high computa-

tional cost of the full ESM, and are therefore limited to anal-

ysis of sulfur burdens and radiative forcings.

In the baseline simulations the eruption took place in the

Tropics. Because the predominant meridional transport in

the stratosphere is from the Tropics towards the poles, sul-

fur released in the Tropics is expected to spread throughout

most of the stratosphere. On the other hand, sulfate released

in the mid- or high latitudes will spread less effectively to

the lower latitudes, and an eruption at mid- or high latitudes

will therefore lead to more local effects in only one hemi-

sphere. Therefore we conducted a sensitivity run simulating

a July eruption during SRM at Mount Katmai (Novarupta)

(58.2◦ N, 155◦ W) where a real eruption took place near the

northern arctic area in year 1912 (simulation SRM Arc July).

The local stratospheric circulation patterns over the erup-

tion site will also affect how the released sulfur will be trans-

ported. Furthermore, stratospheric circulation patterns are

dependent on the season and thus sulfur transport and sub-

sequent climate effects can be dependent on the time of the

year when the eruption occurs. For example, the meridional

transport toward the poles is much stronger in the winter than

in the summer hemisphere (Fig. B1). For this reason, we re-

peated both the tropical and the Arctic volcanic eruption sce-

narios assuming that the eruption took place in January in-

stead of July (SRM Volc Jan and SRM Arc Jan, respectively).

B2 Results from sensitivity simulations

Figure B2 shows that the season of the tropical eruption does

not significantly affect the stratospheric sulfate burden or the

global mean clear-sky radiative forcing (simulations SRM

Volc and SRM Volc Jan). The difference in peak burden val-

ues between the simulations with January and July eruptions

is under 1 % (0.11 Tg (S)) and in peak clear-sky forcing about

1 %. Although the timing of the eruption does not have a

large impact on the global mean values, there is some asym-

metry between the hemispheres as peak value of additional

sulfate from the eruption is 54 % larger after the tropical NH

eruption in July (boreal summer) than in January (boreal win-

ter) (not shown). This is because the predominant meridional

wind direction is towards the south in July and towards the

north in January (Fig. B1). Our results are consistent with

previous studies (Toohey et al., 2011; Aquila et al., 2012)

Month

Latit

ude

(°)

Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct

−80

−60

−40

−20

0

20

40

60

80

v−w

ind

(m/s

)

−0.2

−0.1

0

0.1

0.2

Figure B1. Meridional wind components (positive values from

south to north) at 25 km altitude in CTRL simulation with

MAECHAM5-HAM-SALSA.

which showed that a Pinatubo type tropical eruption in April

would lead to an even increase in AOD in both hemispheres,

while a volcanic eruption during other seasons will lead to

more asymmetric hemispheric forcings. We show that these

results hold also if the eruption takes place during SRM.

On the other hand, if the eruption takes place in the Arctic,

the season of the eruption becomes important. Figure B2a

shows that a summertime Arctic eruption (SRM Arc July)

leads to similar global stratospheric peak sulfate burden as

the tropical eruptions (SRM Volc Jan and SRM Volc), al-

though the burden declines much faster after the Arctic erup-

tion. However, an Arctic eruption in January (SRM Arc Jan)

leads to a global stratospheric sulfate burden peak value that

is only ∼ 82 % of the July eruption value. The peak value

is also reached 2 months later in the January eruption. Re-

garding the global forcing (Fig. B2b), an Arctic winter-time

eruption (SRM Arc Jan) leads to a very similar peak forcing

than the tropical eruptions, while the additional peak forc-

ing (compared to the pre-eruption level) is 38 % lower if the

Arctic eruption takes place in July.

It is interesting to note that in the case of the Arctic vol-

cano, a July eruption leads to a clearly higher stratospheric

sulfate peak burden than the January eruption, but the oppo-

site is true for global peak forcing (Fig. B2). A major reason

for this is the strong seasonal variation in available solar ra-

diation and subsequent hydroxyl radical (OH) concentration

in the high latitudes. OH is the main oxidant that converts

SO2 to sulfuric acid (H2SO4). Due to the rising OH concen-

trations in the Arctic spring, the peak in sulfur burden in the

January eruption is reached during the Arctic summer when

there is highest amount of sunlight available to be reflected

back to space. However, when the eruption takes place in

July, the peak burden is reached already in October due to

high OH concentrations, and thus much faster compared to

the winter-time eruption. However, when the peak value is

reached, the intensity of solar radiation has already dramati-

cally decreased, and thus the peak radiative forcing from the

eruption remains small. The fast conversion of SO2 to sulfate

also leads to larger particles than after the winter eruption



Table B1. Sensitivity scenarios run only with MAECHAM5-HAM-SALSA. Here Jan refers to a volcanic eruption in January and Arc to an

Arctic eruption at the site of Katmai.

Scenario Timing of eruption Eruption site SRM

SRM Volc Jan 1 January Pinatubo (15◦ N, 120◦ E) suspended

SRM Arc Jan 1 January Katmai (58◦ N, 155◦ W) suspended

SRM Arc July 1 July Katmai (58◦ N, 155◦ W) suspended

Figure B2. (a) Stratospheric sulfate burden and (b) global mean clear-sky shortwave radiative forcing after the eruption in January (blue

line) and July (magenta line) and Arctic eruption in January (cyan line) and July (orange line).

and consequently to faster sedimentation and shorter lifetime

(Fig. B2a).

Another main factor that has impact on the climate effects

of an Arctic eruption is the stratospheric circulation. Con-

current circulation patterns can influence the sulfate lifetime

and radiative effects. As Fig. B1 shows, there is a strong sea-

sonal cycle in the Arctic meridional winds. If an Arctic vol-

cano erupts in January, strong zonal polar vortex winds block

poleward transport of released sulfur and it can spread to-

wards midlatitudes. In contrast, in July the atmospheric flow

is towards the north at the northern high latitudes (Fig. B1)

and the sulfur stays in the Arctic. At the same time season-

ality of subtropical barrier affects how sulfate is transported

to the Tropics. As Fig. B1 shows, winds in the northern bor-

der of the Tropics are towards the south only between April

and July and sulfur is transported to the Tropics only dur-

ing this time period. There is clearly more sulfate at the

northern border of the Tropics during these months after the

Arctic eruption in January, while most of the sulfate is al-

ready removed from the atmosphere if the volcano erupted

in July. Thus 6 months after the Arctic eruption, the strato-

spheric sulfur burden in the Tropics between 30◦ N and 30◦ S

is 3.1 Tg (S) for a July eruption but 4.2 Tg (S) for a January

eruption. Since the Tropics have much more solar radiation

for the sulfate particles to scatter than the higher latitudes,

part of the stronger radiative forcing in the SRM Arc Jan

simulation compared to SRM Arc July (Fig. B2b) arises from

this difference in transport to the Tropics. Furthermore, since

the lifetime of sulfur is longer in the low than in the high

latitudes, this leads to a longer average sulfur lifetime in the

SRM Arc Jan simulation (Fig. B2a).



Acknowledgements. This work was supported by the Academy

of Finland’s Research Programme on Climate Change (FICCA)

(project 140867), Academy of Finland’s Centre of Excellence

Programme (decision 272041), Maj and Tor Nessling Foundation

(grant 2012116), and the Academy of Finland’s Academy Research

Fellow position (decision 250348). The authors wish to thank

T. Bergman and S. Rast for technical assistance with the models,

A. Karpecho for helpful discussions related to stratospheric

dynamics and T. Kühn for discussions related to using coupled

models and A. Baran for comments concerning about HIRS data.

The ECHAM–HAMMOZ model is developed by a consortium

composed of ETHZ, Max Planck Institut für Meteorologie,

Forschungszentrum Jülich, University of Oxford and the Finnish

Meteorological Institute and managed by the Center of Climate

Systems Modeling (C2SM) at ETHZ.

Edited by: J.-U. Grooß

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radiative and climate impacts of a large volcanic eruption ... · stratospheric sulfur injections...

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